The present invention relates to a rocker arm for operating a fuel injector wherein the rocker arm is driven by a cam to operate the injector, and more particularly relates to the drive between the camshaft and the fuel injector.
The operation of fuel injectors in the form of a unit injector requires the application of a certain force derived from a camshaft. One type of such unit injectors, commonly known as HPI injectors, incorporates a mechanically driven pump piston which, like a mechanical pump of the displacement type, forces fuel out via an injection nozzle. This type of fuel injector incorporates a nozzle which is normally open during the period when the injector is being replenished, unlike other types of unit injectors which have a nozzle which is closed during fuel replenishment. Injectors with closed nozzles therefore incorporate a separate valve on the nozzle which valve, during injection, opens under the influence of a high fuel pressure caused by the fuel having been pumped up in a separate pump section of the injector to sufficiently high pressure to open the valve. In the case of injectors with open nozzles, i.e. HPI injectors, the pressure increase and the injection are brought about by the same pump piston. In this type of injector it is important for the pump piston of the injector to be correctly set relative to the camshaft so that the pump piston is not subject to too great a force which would press it too powerfully against its seat, with consequent risk of deformation, damage and the like. Possible causes of the pump piston "bottoming" in this manner for reasons other than incorrect setting of the fuel injector and/or its drive train include the occurrence of large temperature differences between the injector and the fuel and/or certain parts having become severely worn.
Where the fuel injector drive is taken from an underslung camshaft, as in U.S. Pat. No.4,471,909, the power-transmitting push rods are inherently flexible enough to be able to absorb elastically the overloads which may occur, without the forces involved being transmitted to the injector and its pump piston. In contrast, where an overhead camshaft is used, where the rocker arm may more directly engage the camshaft without an intermediate push rod, the whole drive train is so rigid that there are no corresponding elements capable of absorbing overloads.
Swedish patent specification SE 501 026 and corresponding U.S. Pat. No.5,647,325 refers to a fuel injection arrangement provided with a rocker arm comprised of a rigid section and a resilient section which is fixed to the rigid section and which deflects at a predetermined load in order to limit the pressure increase resulting from increasing injection volume. It thus refers to an arrangement provided with a metallic resilient section mounted on the rocker arm between the camshaft and the injector, resulting in the rocker arm being totally rigid up to a limit value and beyond which it becomes elastic. That known arrangement solves the problem by increasing the length of the injection period from a given period without increasing the maximum injection pressure and by creating the possibility of reaching the desired pressure even at partial load and low engine speed, which is a different problem compared with the present invention. That known arrangement relates to the type of fuel injector which has "closed nozzle" according to the foregoing description, which means that that arrangement does not risk being subject to overloads in the manner stated above.